The invention relates to a way to prepare stable, chlorine free modified starch which is useful in paper coating and sizing.
Starches, which for the most part are modified starch derivatives, are one of the most used raw material group in paper industry, besides fibres and pigments. Starch is used either as mass starch increasing the dry strength of the paper, whereby it is added to the mass in the wet end of a paper-making machine, or as surface size, whereby the starch solution is spread onto the surface of dried paper. Additionally, starch is used in paper industry as a binder in coating pastes and colours, as a protecting colloid in AKD dispersions, and as a stem in starch based fixatives.
Starch is a carbohydrate present in all plants in nature. It is present in concentrated form in seeds, roots and tubers, where it functions as stored food for the new growing season. Starch is a glucose polymer, in which anhydroglucose units are linked to each other with xcex1-D-glucosidic bonds. Glucose chains are either linear (amylose) or branched (amylopectin). In a normal case the proportion of amylose is smaller than that of amylopectin, about 20 to 25%. The size of a starch chain is also variable, depending on the plant from which the starch has been isolated, or if it is the question of amylose or amylopectin. Amylose has a shorter chain than amylo-pectin.
The sizing properties of starch are based on the large number of its hydroxyl groups, which are able to form hydrogen bonds. In a water solution the hydrogen bonds are formed between hydroxyl groups and water molecules. Thereby the starch binds water by means of hydrogen bonds, which is an important characteristics e.g. in a coating paste and colour. During drying water molecules are removed and hydrogen bonds are formed between starch, fibres and other components used in paper making, whereby the starch functions as a dry strength size in the paper and in coating paste.
Starch is insoluble in cold water. When a mixture of starch and water is heated, starch begins to dissolve, to gelatinize, at a certain temperature, which can be seen as the rise of the viscosity of the solution. Viscosity is raised with continued heating, until individual starch chains. begin to detach from each other, which is observed as a lowering of viscosity. When the solution is cooled, the starch chains begin to associate anew to each other by means of hydrogen bonds, whereby the viscosity again raises. In a dilute solution the associated starch chains settle onto the bottom of the container, but concentrated solutions form a three-dimensional gel. The phenomenon is called retrogradation.
The gelatinizing properties of starch can be studied by a Brabender-viscograph, by the help of which the behavior of starch is detected during heating and cooling. Starch sludge is heated in the device with an even rate up to 95 xc2x0 C., kept there for 15 minutes, whereafter the solution is cooled with an even rate to room temperature. All the time during the determination, viscosity and temperature are measured, and graphs for them are drawn.
Starch solutions have two functional characteristics:
Viscosity: As by polymer solutions in general, the viscosity of a starch solution depends on the average molecular size. In addition to the molecular size, viscosity is effected by temperature, dry matter and the ionic concentration of the solution (pH, hardness, conductibility).
Stability: As concentrated solutions starch has a tendency to retrogradate when cooling, which is detected as rising of viscosity, as turbidity or as solidifying of the solution. Retrogradation is mainly caused by the linear chains, which are easily attached to each other by means of hydrogen bonds. When starch is degraded, the proportion of linear chains increases. The stability increases, when the branching and the number of side chains are increased.
Solutions of natural starches retain their fluidity when cooked and again cooled to room temperatures provided the dry matter concentrations of the solutions do not exceed about 5%. The concentration of dry matter below which a cooked and then cooled solution of starch retains its fluidity is hereby defined as the retrograde concentration.
The modification of the starches can be divided into two main types: chemical and Theological. In this connection the chemical modification means the substitution of starch with chemical groups and by means of them changing the charge condition of the starch, or increasing the stability. Since starch is a polyol, these modifications are mostly ethers or esters. Rheological modifications are made, when higher dry matter contents are desirable in solutions, which means decreasing the viscosity by hydrolysis or oxydation. For hydrolysis, enzymes or acids can be used, and for oxydation, e.g. sodium hypochlorite or hydrogen peroxide. This kind of products include surface sizes and binders of coating paste and colour. In some cases, such as in cationic surface sizes, both modifications are made.
Starch products with low viscosity, used in surface sizing and as binders of coating pastes, are usually prepared by oxidizing with sodium hypochlorite. Besides hydrolyzing starch chains to shorter fragments, hypochlorite forms carboxyl groups which stabilize the product. The stability of the products is very important in modified starches used as binders of coating paste, which starches must be prepared and used as solutions, the dry matter of which is high. The definition xe2x80x9chighxe2x80x9d is used in this connection to mean a dry matter concentration of over 5%, which is not possible to achieve with a starch in natural state so that the solution retains its fluidity. Preferably the dry matter concentration of the starch is between 10 to 15% by weight, which are commonly used values for starch solutions in paper surface size compositions. Even higher concentrations can be used, up to 25% or higher, which concentrations are applicable in coating applications for paper or cardboard. As indicated above, the stability of a starch solution means in this connection such a solution which, while cooling, does not retrograde to a hard gel, but retains its fluidity.
A general objective in pulp and paper industry is to try to decrease formation of organic chlorine compounds (OX) in manufacturing processes, which has led to decreased use of chlorine. AOX-determination (AOX-absorbable organic halogen) is used as the measure of organic halogen compounds, which indicates the amount of elementary halogen bound to organic material. Since in this connection chlorine is the most common halogen, in practice reference can be made to organic chlorine compounds Since starch is a raw material commonly used in paper industry, demands have been presented to use also starch modifying processes where no chlorine is used, as the risk of the formation of organic chlorine compounds is obvious also when oxidizing with hypochlorite. Compounds formed can remain in the product and be carried with the product to the paper mill, which for its part increases the amount of organic chlorine compounds in paper and is detrimental when trying to manufacture OX-free paper.
Two required characteristics for the starches for coating and surface size compositions for paper and cardboard are thus: low viscosity and stability.
Besides by hypochlorite oxidizing, the viscosity can be decreased with other oxidizers, such as hydrogen peroxide oxidation, or by hydrolyzing with an acid. Hydrogen peroxide oxidizing has been used e.g. in U.S. Pat. No. 3,655,644, where viscosity has been decreased using a copper catalyst, without which hydrogen peroxide does not function as a starch degrading agent. In said patent subsequent acetylation is also disclosed. By this process a desired viscosity level for instance for paper coating compositions is obtained, but the stability of the products is insufficient in view of paper coating, when dry matter contents of the starch is raised to the above named levels used for surface size or coating compositions. The products having a this high dry matter content start loosing their fluidity when cooled after cooking, i.e. they start to retrograde.
The retrogradation of a starch thinned by a catalyzed hydrogen peroxide oxidation can be prevented in accordance with the invention so, that the starch is stabilized by combined acetylation and crosslinking after thinning. The proceeding produces cooked starches which retaining their fluidity in cooled solutions even in dry matter concentrations of 25% or higher.
The prior art discloses the publication EP-A-143643 which suggests the combined acetylation and c ross-linking treatment for starch. The treated starch is intended for food industry purposes and the aim is to achieve a starch of high viscosity. The high viscosity presupposes the treatment of a starch with its natural molecule size. By the treatment the viscosity can be retained in the severe temperature and shearing environmets of food processing. In said process the cross-linking effect is achieved by adipic acid, which is a harmfull component in food products.
In order to lower the contents of said component the publication EP-A-143643 suggests a pretreatment with hydrogen peroxide of the starch in order to increase its reactivity towards the adipic acid. A higher reativity means the need of lower amounts of adipic acid for the intended cross-linking effect.
In the starch products manufactured by the process according to the invention, degradation of starch with hydrogen peroxide and copper catalyst, as well as stabilization by acetylation and cross-linking, enables the preparation of modified starch products without chlorine. The products corresponds to products oxidized with hypochlorite. The products according to the invention may be used as binders for coating pastes and as surface sizes in the same way as products oxidized with hypochlorite, which appears from the Examples described hereafter illustrating the invention.
Although potato starch is used as a raw material in the Examples, it is to be appreciated that in the process according to the invention all known commercial starches may be used.